Fix counting and filtering on the dashboard page for issues (#26657)

This PR has multiple parts, and I didn't split them because
it's not easy to test them separately since they are all about the
dashboard page for issues.

1. Support counting issues via indexer to fix #26361
2. Fix repo selection so it also fixes #26653
3. Keep keywords in filter links.

The first two are regressions of #26012.

After:

https://github.com/go-gitea/gitea/assets/9418365/71dfea7e-d9e2-42b6-851a-cc081435c946

Thanks to @CaiCandong  for helping with some tests.

modules/indexer/issues/indexer.go

@@ -13,6 +13,7 @@ import (
 
 	db_model "code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/container"
 	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/indexer/issues/bleve"
 	"code.gitea.io/gitea/modules/indexer/issues/db"
@@ -277,7 +278,7 @@ func IsAvailable(ctx context.Context) bool {
 }
 
 // SearchOptions indicates the options for searching issues
-type SearchOptions internal.SearchOptions
+type SearchOptions = internal.SearchOptions
 
 const (
 	SortByCreatedDesc  = internal.SortByCreatedDesc
@@ -291,7 +292,6 @@ const (
 )
 
 // SearchIssues search issues by options.
-// It returns issue ids and a bool value indicates if the result is imprecise.
 func SearchIssues(ctx context.Context, opts *SearchOptions) ([]int64, int64, error) {
 	indexer := *globalIndexer.Load()
 
@@ -305,7 +305,7 @@ func SearchIssues(ctx context.Context, opts *SearchOptions) ([]int64, int64, err
 		indexer = db.NewIndexer()
 	}
 
-	result, err := indexer.Search(ctx, (*internal.SearchOptions)(opts))
+	result, err := indexer.Search(ctx, opts)
 	if err != nil {
 		return nil, 0, err
 	}
@@ -317,3 +317,38 @@ func SearchIssues(ctx context.Context, opts *SearchOptions) ([]int64, int64, err
 
 	return ret, result.Total, nil
 }
+
+// CountIssues counts issues by options. It is a shortcut of SearchIssues(ctx, opts) but only returns the total count.
+func CountIssues(ctx context.Context, opts *SearchOptions) (int64, error) {
+	opts = opts.Copy(func(options *SearchOptions) { opts.Paginator = &db_model.ListOptions{PageSize: 0} })
+
+	_, total, err := SearchIssues(ctx, opts)
+	return total, err
+}
+
+// CountIssuesByRepo counts issues by options and group by repo id.
+// It's not a complete implementation, since it requires the caller should provide the repo ids.
+// That means opts.RepoIDs must be specified, and opts.AllPublic must be false.
+// It's good enough for the current usage, and it can be improved if needed.
+// TODO: use "group by" of the indexer engines to implement it.
+func CountIssuesByRepo(ctx context.Context, opts *SearchOptions) (map[int64]int64, error) {
+	if len(opts.RepoIDs) == 0 {
+		return nil, fmt.Errorf("opts.RepoIDs must be specified")
+	}
+	if opts.AllPublic {
+		return nil, fmt.Errorf("opts.AllPublic must be false")
+	}
+
+	repoIDs := container.SetOf(opts.RepoIDs...).Values()
+	ret := make(map[int64]int64, len(repoIDs))
+	// TODO: it could be faster if do it in parallel for some indexer engines. Improve it if users report it's slow.
+	for _, repoID := range repoIDs {
+		count, err := CountIssues(ctx, opts.Copy(func(o *internal.SearchOptions) { o.RepoIDs = []int64{repoID} }))
+		if err != nil {
+			return nil, err
+		}
+		ret[repoID] = count
+	}
+
+	return ret, nil
+}